Generally, there are four types of golf balls on the market today, i.e., one-piece balls, two-piece balls, wound balls, and recently, multilayer balls. One-piece balls are solid balls of a homogeneous construction which are typically used as practice or range balls. Two piece balls are formed by molding a cover around a solid core. Wound balls are formed by winding a thin elastic thread around a liquid filled or solid core to form a wound center upon which a cover is molded. Multilayer balls can have a variety of constructions, such as having multiple core layers, an intermediate or mantle layer(s) or multiple cover layers and combinations thereof.
The composition of a golf ball's cover has been found to affect on the overall performance of the golf ball, including such parameters as its distance, spin rate, "click" and feel etc. Historically, balata, i.e., a trans polyisoprene rubber, was employed as a cover material. Balata provides a soft cover with excellent spin and feel characteristics which enable a skilled golfer to control the flight of the ball by imparting desirable spin on the ball. However, although balata covers exhibit enhanced playability characteristics, they lack the durability properties required for repetitive play. Likewise, balata is expensive and requires time-consuming and labor-intensive manufacturing procedures.
In the middle 1960's, E.I. DuPont de Nemours Company, Inc. developed a new species of resins known as ionomers or ionomer resins. These resins are sold, inter alia, under the trademark SURLYN.RTM., and have essentially replaced balata as a golf ball cover stock material. Ionomeric resins are typically ionic copolymers of an olefin, e.g., ethylene, and an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid or maleic anhydride or itaconic acid or anhydride which has been partially neutralized, e.g., with a metal ion. In some cases an additional softening comonomer such as a long chain alkyl acrylate is also included to form a terpolymer. Such ionomers are usually distinguished by the type of metal ions, the amount of acid and the degree of neutralization.
Ionomer resins are effectively crosslinked at "use" temperatures, but exhibit thermoplastic processability at melt temperatures. Ionomer resins typically exhibit a relatively high tensile strength, good clarity, high abrasion resistance, high stiffness and high resiliency.
The early ionomer resins produced extremely durable covers and imparted improved flight distance to the balls. However, these materials were very hard and therefore lacked the desirable click and feel of balata balls. Moreover, because standard prior art ionomer covers are very hard, it is not possible to impart the desirable spin characteristics to the ball required by the more skilled golfer.
It was subsequently discovered that the physical properties of ionomer resins could be modified according to the amount and type of metal ion, the molecular weight, the composition of the base resin (i.e. the nature and relative content of the olefin, the unsaturated carboxylic acid groups, etc.), the amount of acid, the degree of neutralization and the addition of ingredients such as modifiers, softening agents, etc.
Additionally, in order to overcome some of the negative properties of hard ionomer covers, it became well known that ionomer resins can be blended in order to form ionomer cover compositions that provide improved play characteristics. A number of patents have been granted which are directed towards such ionomer blends.
For example, U.S. Pat. No. 3,819,768 to Molitor discloses golf ball covers comprising blends of zinc and sodium ionomers, which overcome cold cracking problems.
U.S. Pat. No. 4,526,375 to Nakade discloses covers comprising a single ionomer in which a portion of the carboxylic acid groups has been neutralized with sodium, while another portion has been neutralized with magnesium or zinc to overcome a cold cracking problem in covers comprising a single ionomer and a fatigue fracture problem in covers comprising sodium/zinc ionomer blends.
U.S. Pat. No. 4,884,814 to Sullivan discloses golf ball covers comprising blends of a high flexural modulus (hard) ionomer and a low flexural modulus (soft) ionomer. The hard ionomers are copolymers of an olefin and an unsaturated monocarboxylic acid, neutralized with sodium or zinc, and have a flexural modulus of about 30,000 to 55,000 p.s.i. and a Shore D hardness of about 60 to 66. The soft ionomers are terpolymers of an olefin, an unsaturated monocarboxylic acid, and an acrylate ester. These terpolymers are neutralized with zinc or sodium, and have a flexural modulus of about 3,000 to 7,000 p.s.i. and a Shore D hardness of about 25 to 40.
U.S. Pat. No. 4,911,451 to Sullivan et al. discloses a golf ball cover including about 10 to 90 percent by weight of an ethylene-acrylic acid copolymer, containing about 11 percent by weight acrylic acid with about 40 percent of the carboxylic acid groups neutralized with sodium, and about 90 to 10 percent by weight of at least one ethylene-acrylic acid copolymer, containing about 11 to 16 percent acrylic acid with about 10 to 40 percent of the carboxylic acid groups neutralized with zinc.
U.S. Pat. No. 5,000,459 to Isaac discloses golf ball covers made from a lithium ionomer resin, wherein up to 50 percent of the lithium ionomer resin is replaced with at least one sodium ionomer resin. The resulting golf balls are found to be comparable in durability and initial velocity to golf balls having a cover formed with lithium ionomer only.
U.S. Pat. No. 5,120,791 to Sullivan discloses golf ball cover compositions containing blends of at least one hard ionomeric resin, preferably an acrylic acid ionomer, and at least one acrylic acid based soft ionomer. Hard ionomers neutralized with sodium, zinc, magnesium, or lithium, having a flexural modulus of 28 to 55 p.s.i. and a Shore D hardness of 60 to 66 are disclosed. The disclosed soft ionomers are sodium or zinc neutralized terpolymers of an olefin, acrylic acid, and an unsaturated acrylate ester monomer, and have a flexural modulus of about 2,000 to 10,000 p.s.i. and a Shore D hardness of about 20 to 50.
U.S. Pat. No. 5,324,783 to Sullivan discloses golf ball covers which include a blend of about 10 to 30 weight percent of soft ionomer resins and about 90 to 70 weight percent of hard ionomer resins. The soft ionomers are terpolymers of an olefin, acrylic or methacrylic acid, and an unsaturated acrylate ester.
U.S. Pat. No. 5,415,937 to Cadorniga et al. discloses a golf ball cover material of a blend of 80-50% of a high stiffness ionomer and 20 to 50% of a very low modulus ionomer ("VLMI") to improve the feel and playability of the ball when compared to a standard ionomer cover, while retaining the distance and resilience of the prior art balls.
However, heretofore, there have not been any disclosures in the prior art directed towards forming golf ball covers and/or intermediate layers from a high crystalline ionomer or a blend of a high crystalline ionomer and at least one additional ionomer.
It has now been found that golf balls with covers and/or an intermediate layer(s) formed from a high crystalline acid copolymer or its ionomer derivatives or a blend of a high crystalline acid copolymer or its ionomer derivatives and at least one additional material, preferably a second acid copolymer or its ionomer derivatives exhibit desirable properties such as increased initial velocity (as compared to "conventional", i.e., non-high crystalline ionomer blends) without any adverse impact on the remaining golf ball properties such as hardness, compression, durability and spin rate. The present application is therefore directed towards golf balls constructed with covers and/or intermediate layers formed from such compositions, as well as methods for making such golf balls.